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Re: A/C really loads down my engine....
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Tue, 24 Jun 2008 05:19:10 Share Post by Email
In Reply to: A/C really loads down my engine...., Bill7, Mon, 23 Jun 2008 15:48:21
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Please post the year and model (V6, 4-cylinder, turbo, non-turbo) because that does impact things.

I'm assuming you have an automatic, because you talk about being in gear at idle.
It's reasonable for the AC to be a big load on the engine at both idle and at low throttle openings (e.g. cruising with a light foot on level highway). That the engine idle bogs down when the compressor kicks in indicates that the idle control is weak. Both the LH and Trionic control boxes get signals from the ACC indicating when the AC kicks in. The engine control then provides a little more throttle and fuel to compensate. If that signal is missing, the engine control won't compensate as well. The Engine control also gets a signal that the automatic is in gear (revers, D, 1,2, 3) and adds a little fuel at low rpm. If that signal is missing, the engine RPM will be lower in gear at idle; add the extra load of the AC, and it gets rough.

It never hurts to check the wiring, but I'll bet that isn't it. There are a few things to check:
The AIC (idle air controller) may be tired and/or dirty. It's used to operating in one range of positions; when the A/C cuts in, it needs to open more. If it's sticky in that range of positions, it won't control airflow well, and the idle will be poor.

The other is to make sure the car is tuned up. Lots of things can cause the idle to be poor, and the extra load of the AC may be just enough to trip it over from OK to lumpy. Check for vacuum leaks. Then check again. If you find one, keep looking. If you have a turbo, make sure the one-way valve between the valve cover and the intake is working. Pull off the big hose and clean the throttle plate with a lint-free cloth. Check the spark plug gap and set it to spec. When the plugs are out, check the color. White plugs mean a lean mixture - either a vacuum leak or a bad FPR. Black plugs mean a rich mixture - might be a bad FPR. Use the right spark plugs; if the car is DI-equipped ONLY use NGKs per the manual. If the car has a distributor, replace the cap and rotor if they look in any way worn, and the spark plug wires if they're more than 3 years old or look damaged. Make sure the air and fuel filters are fresh. If you have a distributor, check the timing.

Doing the tune-up, if you haven't already, will help with the gas mileage. And may fix the bad idle.

Also - how are your motor mounts? If the mounts are shot, if the engine rpm drops even a little into a resonance range, it'll shake like a paint mixer.

posted by 192.249....
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